What term refers to broken rock placed around piers and abutments to prevent erosion and scour?

• A. Gabions
• B. Ballast
• C. Rip Rap
• D. Sag

Answer: (C)

Protecting pier foundations from erosion and scour is achieved with riprap—the layer of broken rock placed around bases to shield them from flowing water and prevent displacement of soil at the substrate.

Riprap works by dissipating hydraulic energy and increasing bed roughness, which reduces near-base flow velocity and shields the concrete or hollow piers and abutments from scour channels developing at the foundation. The stones are typically angular so they interlock and resist movement, and their size and thickness are chosen based on the expected flow velocity, scour depth, and site conditions. Sometimes a toe protection and a graded, layered arrangement are used to ensure stability and long-term performance.

Gabions are wire cages filled with rock and serve a similar protective purpose but in modular units; ballast refers to coarse rock used to support and stabilize structures like railway beds; sag isn’t a standard term for this application.